Billet cut-off control

ABSTRACT

Metal billets are cut from heated logs emerging from a furnace by a shearing system controlled by information detected as the logs emerge from the furnace. Detection in this position eliminates the variations inherently present in the length of the logs resulting from temperature gradients within the furnace, and from a length of time the logs are subject to these gradients. The advancing end of an isolated log is sensed by an electric eye, and the interface between one log and the following log is detected by an eddy current device. The data from these sensing devices is supplied to a computer, which controls the shearing operation so that billets can be cut with a minimum of waste. The billets can be of two segments, and the equipment establishes cutting operations that produce the combined billet length, regardless of the position of the interface between the logs.

BACKGROUND OF THE INVENTION

The extrusion of aluminum is usually preceded by heating a piece,referred to as a "billet", to the point that it is sufficiently plasticto be squeezed through an extrusion die. The die will have an openingdefining the cross-section of the extruded piece. These billets are cutfrom elongated cast "logs", which are cylindrical, and commonly from sixto ten inches in diameter. These are perhaps initially twenty feet long.One approach to supplying the heated billets is to heat the entire log,and cut off the billets as needed as they emerge from a furnace.Furnaces and cut-off equipment for performing this function have beenavailable for some time. The long "tunnel" furnaces vary somewhat indetailed design, but usually have a conveyor system carrying the logsinto one end and through the furnace, and out to the station where thehot billet is sheared from the log. A set of gas burners is usually theheat source, and these are mounted near the discharge end of thefurnace. The products of combustion move toward the entrance end,causing this section of the furnace to form a pre-heat zone.

The billet supplied to the extruder does not need to be in one piece,since it will be merely a non-defined mass of plastic material in theextrusion process. The length of the billet is determined by thequantity of material required for the particular extrusion operation,and the diameter of the log. A particular extrusion operation mayrequire billets twenty four inches long at the available log diameter.This length can be made up of one segment fourteen inches long, andanother ten inches long. When the trailing end of the log approaches theshearing station, either an operator or automatic equipment must causethe system to make up segments of the required length, making allowancefor the fact that the segments have a minimum length determined by thecharacteristics of the conveying and cut-off systems.

The standard approach to controlling the cut-off system is via a controlof the incremental conveyor, or "pusher", that moves the logs throughthe furnace. This, in turn, has been through sensing equipment locatedat the entrance end of the furnace, and which records the position ofthe beginning of the log and of each incremental movement through thefurnace. If the log were of a constant length (minus the length ofpieces cut off), this control system would be adequate. However, thereis a significant temperature gradient along the furnace, and the logsmay remain in one section of the furnace for varying periods of time.They may remain at the hotter outlet end for an extended period, forexample, as the needs of the extruders supplied by the furnace vary forone reason or another. The uncut length of a log may easily vary acouple of inches during its passage through the furnace, which would beenough to throw off the billet lengths selected for best extrusionoperation. The problem is inherent in a system controlled by sensingequipment at the entrance end of the furnace.

SUMMARY OF THE INVENTION

The present invention provides a method of controlling the cut-offoperation as a function of conditions sensed at the exit end of thefurnace, and between that position and the plane of the shearingoperation. These conditions are sensed as the logs are moving into theshearing station. The pusher records the amount of the advance of thelog from the preceding shear. Non-contacting sensing devices record thepassage of the end of one log and the beginning of the next. This ispreferably detected by an eddy current device, and the beginning of anisolated log (with no log preceding it) is preferably also detected byan electric eye. The eddy-current detector responds to the discontinuityin the abutting ends of the logs, which is sensed in the same mannerthat cracks can be sensed in structural members. This is important, asthere is usually little or no gap between the abutting ends of logs, dueto the action of the pusher of the conveyer system. Information derivedfrom this sensing equipment is fed into a conventional computer, whichis programmed to operate the shearing system as a function of this data.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing the general layout of aconventional furnace and shear system, and illustrating the position ofthe detecting system provided by this invention.

FIG. 2 is a schematic view in elevation of that portion of FIG. 1containing the shear system and the detection equipment.

FIGS. 3 and 4 are schematic illustrations showing the conditions in theheated log which are detected by the system, and which are referred toin order to cut billets with a minimum of waste.

DESCRIPTION OF THE REFERRED METHOD

In FIG. 1, the tunnel furnace generally indicated at 10 is usuallyequipped with a pusher conveyor 11 that takes logs in the position shownat 12 in dotted lines, and advances them into and through the furnace.The logs will pass through the pre-heat zone and the heating zone, insequence, where they emerge from the end of the furnace at a temperaturesuitable for the operation of an extrusion machine. A hydraulic shearingdevice 13 is spaced from the exit end 14 of the furnace, and receivesthe logs moving down the conveyor 11 as they emerge in the heatedcondition. The heat is supplied by burners located in the heat zone fedby the manifold and controls generally indicated at 15. The furnacestructure thus far described is conventional.

Detection devices indicated at 16 are positioned between the end of thefurnace and the shearing device 13. This detection system includes theelectric eye unit 17 and the eddy-current detector 18. Both of these aremounted at one side of the path of the logs emerging from the furnace.The electric eye may respond directly to a light beam, or from a mirrormounted at the opposite side of the log path. In either case, the lightbeam must be positioned to be intersected by the moving logs. The eddycurrent detector 18 is of standard design, and is usually equipped witha toroidal probe that is positioned half and inch to an inch from theposition of the side of the moving logs. These units generate a toroidalelectro-magnetic field, producing eddy currents in the material movingadjacent the end of the probe. Variations in the condition of the fieldare detected by the instrument, and these field conditions will varyaccording to whether or not metal passing in front of the probe iscontinuous or discontinuous. Such instruments have been developed to ahigh degree of accuracy for the detection of cracks in structuralmaterials. In the present application, this principle is employed todetect the presence of a plane of discontinuity between one log and thelog that is pressed against it by the pusher system. A device of thistype that appears to perform well in this equipment is being marketed byFaAA Products Corporation under the trademark SMART EDDY. The detectionsystem includes the probe and a computer programmed to suit the needs ofthe particular installation. This company is located at 149 CommonwealthDrive, in Menlo Park, Calif.

In FIG. 3, a schematic relationship is illustrated between a log 19moving through the furnace, and what remains of a preceding log 20passing through the shearing station. The interface 21 represents thetrailing end of the log 20 and the leading end of the log 19. The planeof operation of the shearing device 13 is indicated at 22, and theposition of the eddy-current detector at 23. The electric eye positionis shown at 24, both of these devices being within the space 25 betweenthe end of the furnace and the shear plane 22. With the position of theinterface 21 shown in FIG. 3, the shearing device can cut off a billetof the required length 26, followed by the advancement of the logs 19and 20 after the shear has retracted. The eddy current detector hasalready recorded the passage of the interface 21 as the logs had beenmoved into the FIG. 3 position. The logs can then be advanced the normalbillet length, where the shear will cut at a point which will include apiece from the end of the log 19. If this piece exceeds the minimumlength that the conveying systems and the shear can handle, thiscombined billet consisting of two components is handled in approximatelythe same way as a solid billet.

FIG. 4 represents a different condition which must be accommodated bythe control system. The operation of the shear to cut off the billetlength 26 from the log 20 is followed (after retraction of the shear) bythe advancement of the logs. During this advance, the interface 27 willpass the eddy current detection station 23, which will establish thatthe cutting of a solid piece from a log 20 would leave too short a pieceto be handled effectively by the conveying and shearing system. Thisinformation will cause the program of the computer to advance the logsperhaps half the usual billet length. The log 20 is sheared at thispoint. This piece is pushed on into the storage station shownschematically at 28 in FIGS. 1 and 2, where it is kept hot until amatching piece can be provided. The logs 19 and 20 are then advanced onebillet length, where a cut is made producing a sufficient added lengthfrom the log 19 which (when added to the remaining piece from the log20) will produce a billet of the required length. Another piece is thencut from the log 19 which, when added to the piece now in storage, willproduce a billet of the required length. These are then handled togetheras a new billet. The beginning of an isolated log (which was notpreceded by another log) has been previously detected by the electriceye, with this information fed into the computer for establishment ofthis reference position. The computer is also properly programmed topush the logs back into the furnace after the shearing operation tomaintain the temperature until another billet is called for. The nextadvance of the log end out of the furnace is detected by the electriceye.

Experience has shown that the placement of the eddy current detector 23should be at a distance ahead of the shearing plane by an amountcorresponding to one billet length plus one billet diameter, all dividedby two. A minimum distance resulting from this expression is preferablyheld to approximately twenty inches. The computer device used with thisdetection system is not shown, and is standard. It is programmed to fitthe particular installation. This equipment is capable of producingbillet lengths to a tolerance of zero to plus one-eighth of an inch,which is well beyond the capacity of conventional systems.

We claim:
 1. A method of controlling a cutting system adapted to cutfrom a metal log, emerging from a furnace having an outlet end, billetsof one or more segments having a predetermined total length, each ofsaid segments having a predetermined minimum length, said methodcomprising:presenting an end-for-end sequence of metal logs to saidcutting system at a position spaced from said furnace end on a conveyorestablishing a path of log movement, each of said logs having a leadingand a trailing end; detecting at a position interposed between saidfurnace end and said cutting system, the discontinuity represented by atrailing end of one of said logs and a leading end of the next followinglog, said one and next logs remaining in end-to-end contact; detecting aleading end of a log isolated from any preceding log; and controllingsaid cutting system and advancing said logs as a function of thepositions of said discontinuity and leading end, said leading enddetection being disposed at a distance from said discontinuity detectionless than one log diameter.